For fishing with the use of flies or other lures, a length of line is commonly knotted between the end of the fishing line and the hook to suit the needs of the moment. Short of this, it would be necessary to continuously adapt the size, weight and length of the entire main line to the particular requirements; this would require a large stock of coil supplies. The length of fishing line knotted in between is called leader. It is about 3 to 10 feet long and is prone to heavy wear. But in case of a ground hook-up, the leader is expected to tear so that the expensive main line is saved, if possible with all its length and over an extended period of time. A frequent change of the leader size may become necessary, depending on the sensitivity of the fish.
It will be seen that the leader must be changed at frequent occasions. In the past, it was customary to carry along one loose reel for each line size; often loops came off inadvertently, providing considerable disorder in the fishing equipment containers. Moreover, there is anyhow little space available for storing the fishing accessories, expecially for fly-type fishing.